The invention relates to a disc-shaped information carrier, for example a laser video disc (VLP) or digital audio disc (CD), which is read optically in reflection, provided with an annular optically readable information containing zone which is bounded by a concentric inner zone and with a concentric outer zone extending to the outer edge of the disc, the said information carrier comprising a transparent substrate which as a readout side and an opposite side to the readout side which is provided with a reflective coating.
Disk-shaped information carriers which are read optically in reflection are available commercially, for example, as compact discs which contain digital audio information,. Discs containing video signals (VLPs) and discs containing data (CD-ROMs) are also known. In addition, writable information carriers are also known in which information can be written into the reflective coating by means of a radiation beam. Such discs are used as optical storage discs.
In such disc-shaped information carriers the substrate often consists of a transparent plastic which is given a reflective coating. The plastic may consist, for example, of polycarbonate or polymethylmetacrylate. The polycarbonate discs are made in an injection molding or injection-compression molding process. In the process the actual information, or in the case of writable discs the track information or the like, is imprinted into the disc surface in the form of a microstructure on at least one side. Alternatively, it is also possible to use a flat substrate which is provided with a lacquer which hardens when irradiated with UV light and which, when still in the liquid state, is brought into contact with a die and, while hardening, takes on the microstructure present in the die. In order that the micro-information structure can be read out in reflection, the disc is coated on the back with a reflective layer.
In the mass production of the information carriers, the dies displaying the micro-information structure are usually electroplatings derived from a so-called master. Plastic dies may also be used. For the sake of simplicity, we shall hereinafter discuss only the platings. For the master, a glass disc, which is coated initially with a photoresist layer, is frequently used as the substrate. This photoresist layer is exposed by means of a laser beam focussed on the layer and modulated with the information and then developed, the exposed parts usually being removed from the photoresist layer. The finished master is made electrically conductive, and then a first plating (father plating) is drawn from the master in an electrolytic bath. This father plating can be used immediately as the stamper. Usually, however, a further plating (mother plating) is first of all drawn from the father plating and further platings (daughter platings) are taken from the mother plating, and only these daughter platings are used as stampers.
In the manufacture of such disc-shaped information carriers, very high demands have to be placed not only on the accuracy of the micro-information structure to be transferred to the disc but also on the homogeneity of the substrate material and the defect-free condition of the coating surfaces.
As practice shows, such defects cannot as a rule be eliminated so that a fairly substantial portion of the production has to be rejected. Particularly critical in this context is the outer zone of the information carrier because during manufacture, inhomogeneities, in the form for example of fine air bubbles, readily occur in the outer edge areas of the material. This can occur, for example, when the information carriers are produced by injection or injection-compression molding due to the fact that during pressing the injected material does not cool uniformly in the radial direction. It is also impossible sometimes when handling such discs during the manufacturing process to avoid surface scratches which then also often occur in the outer zone of the disc surfaces. One of the drawbacks in this situation is that, when there is a metallic reflective coating present, the mirror effect of this coating causes such defects to stand out particularly prominently to the observer.